The present invention relates to a conditioning process involving the coating in bitumen of radioactive waste constituted by ion exchange resins.
The bitumen coating of radioactive waste is generally performed by mixing an aqueous suspension of the radioactive waste with fluidized bitumen, followed by the evaporation of the water of the suspension and pouring the thus obtained mixture into a container in order to solidify it.
French Pat. No. 1,315,162, filed on Dec. 6, 1961 by the Commissariat a l'Energie Atomique, describes a bituminizing process of this type, according to which the suspension of the waste is mixed with fluidized bitumen in the presence of a surfactant, which facilitates the separation of the water contained in the separation. Most of this water is then separated either by decanting, or by means of a mechanical device and the product obtained after separating the water is then mixed at a temperature to ensure that it is sufficiently fluid and is finally poured or cast in such a way that cooling leads to solid blocks having an appropriate plasticity.
French Pat. No. 2,052,093 filed on July 15, 1969 by the Commissariat a l'Energie Atomique described an installation for coating a suspension of radioactive products with bitumen using a film evaporator and a gear pump for extracting the coated products obtained at the bottom of the evaporator.
French Pat. No. 2,356,246 describes an improvement to the processes for coating radioactive waste with bitumen consisting of subjecting the waste to a pretreatment by salts, such as calcium and barium chlorides, in order to improve the resistance to leaching of the solidified products obtained. In this case, the treated waste is constituted by evaporation concentrates and/or precipitation sediment comprising salts, such as sodium sulphates and carbonates.
Installations for the continuous coating of radioactive waste suspensions are also known, which use a twin or four screw extruder, in which the waste is mixed with bitumen and the suspension is dried.
Generally, before coating the suspension of the waste, the latter undergoes a neutralization treatment using soda. Thus, it is inadvisable to bitumen coat acid suspension, particularly free nitric acid, in order to obviate any risk of deterioration of the bitumen at the end of the operation when, after evaporating the water, the bitumen is in the presence of concentrated acid. Moreover, neutralization before bitumen coating limits corrosion to the installations.
When the radioactive waste is formed by organic ion exchange resins, particularly of the anionic type in form OH.sup.- and/or Cl.sup.-, the performance of the bitumen coating processes does not make it possible to obtain an adequate treatment capacity of the bituminizing installation. Moreover, the bituminous coatings obtained suffer from the major disadvantages of swelling when subsequently immersed in water. In this case, the volume increase of the coatings can reach 20% and even exceed 100% in certain cases, which leads to a disintegration of the coated waste.
Processes for conditioning ion exchange materials in thermosetting resins or in cement are also known of the type described in French Pat. No. 2,361,724 and Japanese Pat. No. 48-28899. According to these processes, in order to obtain crack-free conditioned products, it is necessary to pretreat the ion exchange resins to replace the H.sup.+ ions by other cations, in order to ensure that the resins do not fix certain of the reagents necessary for obtaining the setting of the thermosetting resin or the cement.
German Pat. No. 3,102,473 also describes a process for pretreating a mixture of cation and anion exchange resins, which then makes it possible to separately condition the resins by incorporation in bitumen or cement. According to this patent, in order to separate the cation exchange resins from the anion exchange resins, the resin mixture is contacted with an aqueous solution of a salt, such as an alkali metal acetate, nitrate, chloride or sulphate, in order to replace the H.sup.+ and/or Na.sup.+ ions of the cationic resins by sodium and the OH.sup.- ions of the anionic resins by other anions. The resins can then be separated by means of a liquid, whose density is between that of the grains of one type of resin and that of the grains of the other type of resin. Thus, this patent does not deal with the problem of swelling in water on the part of the bituminous coatings.